It is well known (J. E. MARECHAL--VI Petroleum Congress section III paper 1 PD 7), that the cold flow problems of the petroleum cuts and gasoils in particular, are related to the content of long chain linear paraffins (long n-paraffins in these petroleum cuts). The hydrocarbon petroleum cuts treated according to the process contain paraffins or are themselves pure or mixed paraffins. By way of example, the gasoils of direct distillation contain a large proportion of C.sub.12 -C.sub.23 n-paraffins. The heaviest cuts may contain longer chain C.sub.12 -C.sub.30 paraffins, for example. These hydrocarbon petroleum cuts can thus be gasoils whose initial distillation point is generally at least equal to 150.degree. C. and the final point, normally fixed at 450.degree. C., can reach 530.degree. C. when the cuts are obtained by distillation under vacuum.
In the latter case, it is possible to obtain lubricants from these cuts provided there is a previous treatment to eliminate the aromatic compounds by hydrorefining or extraction by solvent. The problem thus consists in lowering the flow point of the product obtained, without too greatly diminishing its viscosity index, while ensuring a suitable yield to the operation. The flow point of lubricants is defined by standard AFNOR T 60105.
The gasoils, in order to be of a commercial quality, must satisfy the specifications of motor gasoils and domestic fuels. From this point of view, it can be noted that the most restrictive characteristics are the flow characteristics and the sulfur content.
The most currently used flow characteristics for the gasoils are the flow point and cloud point (C.P.) defined by standard AFNOR T 60105 T and the limit temperature of filterability normally designated by the sign (LTF)defined by standard AFNOR N 07.02.
Generally, the gasoil cuts of direct distillation must be made to conform to the specifications by various appropriate treatments.
Two types of solutions have been proposed in order to improve the flow characteristics of the petroleum cuts.
The first solution consists in adding adjuvants. The second solution consists in applying to them a catalytic hydrotreatment called "dewaxing".
When the sulfur content is too high, a desulfurization treatment is generally carried out in the presence of hydrogen. The catalytic hydro desulfurization (CHD) made possible by contact with a cobalt-molybdenum type catalyst supported by alumina. In the present invention, the hydrodesulfurization is ensured by the claimed catalyst.
Certain dewaxing processes use cobalt-molybdenum type hydrodesulfurization catalysts deposited on acidic supports. These processes, consume hydrogen and provide low yields of gasoils, are not the most economically interesting.
Other dewaxing processes are known. The processes use platinum-based catalysts deposited on halogenated alumina or alumina-silica or use zeolite based catalysts which may contain precious metals.
The platinium catalysts deposited on halogenated alumina or aluminasilica necessitate operating conditions (temperature, pressure, spatial velocity) that in certain cases render the process difficult to integrate economically with the hydrodesulfurization which, in a classical refining installation, remains necessary.
Zeolite based catalysts have featured in numerous patent applications over the last decade. Examples of zeolite based catalysts are disclosed in French Pat. Nos. 1.496.969 and 2.217.408, U.S. Pat. Nos. 3,663,430, 3,876,525 and 3,578,399.
Zeolites described in these patents belong to the family of mordenites with an admixture of precious metals, especially platinum or palladium, or zeolite Z SM 5 of the company MOBIL or further belong to the family of offretites (European Pat. No. 16.530 in the name of MOBIL).